1. Field of the Invention
This invention relates to an auto tensioner for automatically keeping the tensile force of the belt of a belt drive mechanism at a suitable level and more particularly to an auto tensioner of the type in which a tension pulley around which the belt is passed is arranged at an offset position with respect to a support shaft for supporting a tension pulley supporting arm.
2. Description of the Prior Art
For example, the auto tensioner disclosed in the Unexamined published Japanese Patent Application No. H2-253035 is provided with a tension pulley around which a belt is passed, an arm for supporting the tension pulley, a support shaft for rotatably supporting the arm, a friction plate for applying a rotational resistance to the arm and a coil spring for pressing the arm against the friction plate and urging the arm in a direction of rotation thereof. This auto tensioner regulates the movement of the tension pulley in such a manner that the vibration or impact coming from the belt is attenuated while permitting the movement of the tension pulley, around which the belt is passed, in accordance with the fluctuation of the tensile force of the belt.
The above-mentioned friction plate is sandwiched between the front end face of the boss of the arm and a guide plate integrally attached to the front end of the support shaft so as to attenuate the vibration or impact from the belt. Further, the friction plate is held in pressure-contact with the boss of the arm and the guide plate due to the extension restoring force of the torsion coil spring. Such friction plate is usually formed of a clutch facing material, a brake lining or brake padding material.
In the above-described auto tensioner, the tension pulley is arranged at an offset position with respect to the support shaft so that the load applied on the tension pulley through the belt acts as a moment for tilting the arm. Consequently, the load (unbalanced load) applied on the required angle zone on the circumference of the friction plate excessively increases. Therefore, the abrasion of that zone tends to become extremely quick as compared to that of the remaining zone and as a result, the unbalanced abrasion of the friction plate takes place earlier thereby shortening the service life of the friction plate.